Such a device includes: at least one ultrasonic sensor, which radiates into, or receives from, the pipeline ultrasonic measuring signals at angles of incidence and emergence, respectively; and a control/evaluation unit, which ascertains, on the basis of the ultrasonic measuring signals, volume- and/or mass-flow of the medium through the pipeline.
Ultrasonic flow measuring devices of the above-described kind are used often in process- and automation-technology. They make possible contactless ascertainment of volume- and/or mass-flow of a medium in a pipeline. Clamp-on flow measuring devices working according to the travel-time difference method are described, for example, in EP 0 686 255 B1, U.S. Pat. No. 4,484,478, or U.S. Pat. No. 4,598,593. Also, clamp-on flow measuring devices are available from Endress + Hauser under the mark, PROSONIC FLOW.
In the travel-time difference method, the different travel-times of ultrasonic measuring signals in the direction of flow, and opposite to the direction of flow, of the medium are evaluated. From the travel-time difference of the ultrasonic measuring signals, the flow velocity, and, therewith, in the case of known diameter of the pipeline, the volume flow, e.g. volume flow rate, and, in the case of known, or measured, density of the medium, the mass flow, e.g. mass flow rate, are ascertained.
When using the Doppler principle, ultrasonic measuring signals of predetermined frequency are coupled into the flowing medium, and, upon being reflected in the medium, are evaluated. On the basis of a frequency shift arising between the inwardly radiated, ultrasonic measuring signal and the reflected, ultrasonic measuring signal, then the flow velocity of the medium, or the volume- and/or mass-flow, can be ascertained.
Use of flow measuring devices working according to the Doppler principle is only possible, when air bubbles or impurities are present in the liquid medium, so that the ultrasonic measuring signals can be reflected thereon. Thus, use of such ultrasonic flow measuring devices is quite strongly limited, in comparison to ultrasonic flow measuring devices working e.g. according to the travel-time difference principle.
In the case of ultrasonic flow measuring devices working according to the travel-time difference method, the Doppler method, or the cross correlation method, the ultrasonic measuring signals are coupled at a predetermined angle into and out of the pipeline, or measuring tube, in which the flowing medium is located. In order to achieve an optimal impedance matching and, therewith, optimal in- and out-coupling, the ultrasonic measuring signals are coupled into and out of the pipeline or measuring tube via a coupling shoe, or coupling wedge. Principle component of an ultrasonic sensor is at least one piezoelectric element, which produces and/or receives the ultrasonic measuring signals in a defined frequency range.
The ultrasonic measuring signals produced in a piezoelectric element are guided via a coupling wedge, or coupling shoe, and via the tube, or pipe, wall, into the fluid medium. The medium is either a liquid or a gas. Since velocity of sound varies relatively strongly from medium to medium, refraction of the ultrasonic measuring signals is experienced at the interface between two different media. The particular angle of refraction can be calculated from Snell's law, according to which the angle of refraction depends on the ratio of the propagation velocities of the two, mutually bordering media.
In- and out-coupling of the ultrasonic measuring signals is especially problematic, when the pipeline is manufactured of metal and a gaseous medium is flowing in the pipeline. Since the acoustic impedance of a metal and a gas usually differ by an order of magnitude, a large part of the ultrasonic measuring signals are reflected back at the interface, both in the case of the in-coupling and in the case of the out-coupling. The back-reflected fraction is so large, that no reliable flow measurement is possible with a conventional ultrasonic sensor. If, then, yet other sources of error arise, which relate, for example, to the installation and mounting or to changes of environmental conditions, then a conventional ultrasonic flow measuring device is unsuitable for the particular application.
Clamp-on ultrasonic sensors for ultrasonic flow measuring devices require, for the in- and out-coupling of the ultrasonic measuring signals, a direct contact with the pipeline. Especially, no additional interface of gas, or air, must be located between the ultrasonic sensor and the pipeline. The in/out coupled power is, at comparable power density, greater, the greater the contact surface between the ultrasonic sensor and the pipeline. For improving the efficiency of in- or out-coupled power transmission, it is known to match the contact surface of the ultrasonic sensor to the surface of the pipeline, or to provide paste, or plastic, coupling media between the ultrasonic sensor and the pipeline. Solutions involving suitable plastic films placed between the ultrasonic sensor and the pipeline likewise belong to the state of the art.
As already mentioned, in the case of pipelines having smaller nominal diameters, or small outer diameters, only a fraction of the ultrasonic measuring signals available for in- or out-coupling are actually in- or out-coupled, when the contact surface between ultrasonic sensor and pipeline is flat. If a curved contact surface matched to the pipeline is used, then the ultrasonic measuring signals are, it is true, in- and out-coupled with a higher efficiency, but then the ultrasonic sensor is, because of the special design, only optimally fitted for the particular nominal diameter, or particular outer diameter, of the pipeline. This means that, for each nominal diameter, or each outer diameter, of pipeline, a specially fitted ultrasonic sensor must be constructed.